A transmission line of the microstrip type is described in the publication entitled: "Properties of Microstrip Line on Si-SiO.sub.2 System" by HIDEKI HAZEGAWA et al. in "IEEE Transactions on Microwave Theory and Techniques, Vol. MMT-19, No. 11, November 1971, pp. 869-881".
According to the above document a microstrip line consists of a piled structure formed by a metallic layer used as a ground plane, a silicon (Si) semiconductor layer, a thin silicon dioxide (SiO.sub.2) dielectric layer and a metallic strip having a given transverse dimension.
This document describes that such a line permits propagation in three fundamental modes. The first mode is a "quasi-TEM mode", the second is a "skin-effect mode" and the third is a "slow-wave mode".
The larger the resistivity of the semiconductor layer the more the propagation mode approaches a conventional TEM mode.
The third mode termed "slow-wave mode" appears when the operating frequency is low, of the order of 10 to 10.sup.3 MHz, and when the resistivity of the semiconductor layer is also low, of the order of 10.sup.4 or 10.sup.2 .OMEGA..cm. In this "slow-wave mode" the magnetic energy is distributed over the semiconductor layer whereas the electric energy is stored in the dielectric layer. The sum of these energies is transmitted perpendicularly to the layers through the thin silicon dioxide (SiO.sub.2) dielectric layer. The phase velocity thus diminishes due to the energy transfer to the interface of the semiconductor and the dielectric (Si/SiO.sub.2).
The phase constant is expressed in terms of normalized wavelengths: .lambda..sub.g /.lambda..sub.o, which ratio is equal to the propagation velocity in the line divided by the velocity of light in free space. The maximum usable frequency largely depends on the resistivity of the semiconductor layer and becomes highest when the resistivity reaches 10.sup.-1 .OMEGA..cm, while this frequency remains below the GHz domain.
Alternatively, the phase constant and the characteristic impedance of the line also very much depend on the transverse dimension of the strip and the thickness of the semiconductor layers and the dielectric separating the ground plane from the strip.
In conclusion, this document describes that the operation in the slow-wave mode presents high losses which could be diminished by devising a multi-layer structure between the ground plane and the strip, this multi-layer structure being formed by an alternation of semiconductor layers and thin dielectric layers so as to reduce the losses caused by the skin effect. If such a multi-layer structure were used for realising a microstrip line operating in the slow-wave mode, then the dimension of the line could be reduced which would permit reducing the dimensions of the integrated circuits with the line functioning in the GHz frequency domain or lower frequency domains.
A technical problem which is currently posed is the monolithic integration of the microwave circuits on a semi-insulating substrate. In effect, if a microwave circuit is not integrated monolithically, it performs less well because of losses in the inter-substrate connections, it functions at less high frequencies owing to parasitic capacitances that appear, it has a larger consumption, and it is more expensive because it requires larger semi-insulating substrate surfaces and more manufacturing stages.
The prior-art transmission lines necessary for realising microwave circuits, for example, the microstrip lines operating in the quasi-TEM mode have to date covered a considerable surface of the substrates and made monolithic integration difficult because the circuits have become complex.
The technical problem of monolithically integrating Microwave Integrated Circuits (MIC's) cannot be solved until the problem of miniaturisation of the transmission lines is solved, while considering the fact that it must be possible to fit in the miniaturisation with the manufacture of the other elements of the circuit, for example, transistors and interconnecting lines, and the fact that the losses in the lines are not to augment and that the operating frequency is to be that of the microwave circuits.
The prior-art arrangement does not meet these requirements. In effect, either it operates in the quasi-TEM mode and in this case the dimensions of the lines are too large, or it operates in the slow-wave mode with the advantage of a considerable phase shift and smaller dimensions, but in that case there are the following disadvantages:
the frequency domain investigated is too low and not compatible with the MMIC's;
the substrate presents too low a resistivity which is not compatible with the realisation of other elements of the MMIC's, or which at least bounds their performance;
the generation of the slow waves depends very much on the resistivity of the substrate, which entails that the doping of the substrate has to be considerably optimized. This optimization adds to the manufacturing costs of a circuit including such a line, whereas there is nevertheless a risk of dispersion when the circuit is in operation;
the arrangement formed by the line makes a ground plane necessary at the back of the substrate, which leads to technological problems with regard to realisation of the interconnections;
the operation losses in the slow-wave mode with a single semiconductor layer are very high; and
if one wishes to diminish the losses in order, to benefit from the advantage presented by the slow-wave lines because their dimension is diminished, the manufacturing technique of the substrate that includes alternate semiconductor and dielectric layers renders the arrangement harder to realise, more costly, and less compatible with monolithic integration.
Thus, from the information the in the above article it appears that the lines operating in the slow-wave mode are eligible for realising monolithic integrated circuits because their dimensions could be minimized relative to those lines operating in the TEM mode or the conventional quasi-TEM mode, but, on the other hand, that their operating domain, their performance, and manufacturing technology is incompatible with that required for the MMIC circuits.
It is an object of the present invention to propose a slow-wave transmission line of the MICROSTRIP type in which the propagation structure is fully compatible with the integrated circuits, for example, with microwave integrated circuits and, more specifically, with MMIC's.
For this purpose it is an object of the invention to propose a slow-wave transmission line of the MICROSTRIP type whose characteristic features are independent of the characteristic features of the substrate.
It is an object of the invention to propose such a line without a ground plane on the back surface of the substrate.
It is an object of the invention to propose such a line of which the losses are not higher than those of the microstrip lines operating in the TEM mode or conventional quasi-TEM mode.
It is an object of the invention to propose such a line whose dimensions are several times smaller than those of the lines operating in the conventional TEM mode or quasi-TEM mode for identical characteristic features of the line.
It is an object of the invention to propose such a line capable of being connected to microwave circuits.
It is an object of the invention to propose such a line whose manufacturing process can be fully combined with the manufacturing processes of any conventional integrated circuit no matter what the selected semiconductor substrate selected for this circuit is, without increasing the number of steps necessary for the processes, using only layers or materials used in such processes.